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This article explores the connection of the molecular evidence of evolutionary relationships between the various hominins with fossil evidence from the Plio-Pleistocene times, a period covering episodes of immense swings in temperature and rainfall, beginning about 5.5 million years ago (mya) and ending only about 11 kya. New technologies, allowing analysis of trace amounts of genetic material from human fossils, may help determine these relationships such as mitochondrial DNA indicates that widely-dispersed human groups apparently split into at least two descendant populations in the late Middle Pleistocene, perhaps during the period of global climate extremes 480-425 kya. The principle of a molecular clock underlies the use of DNA sequences or their derivatives as cellular fossils to aid reconstruction of speciation events. The estimation of mutation rates for nuclear (chromosomal) DNA is difficult because chromosomes, and the DNA which composes them, recombine and homogenize DNA sequences in every new generation, in addition to undergoing single base substitutions. The overall mutation rate for mitochondrial genes is about 2% every million years. Molecular dates using nuclear gene data support an early Miocene divergence for Asian and African apes at 18 mya, but mitochondrial timescales are younger, by 4–5 million years. It appears that the African great apes are the closest genetic relatives of humans, with estimated splits based on fossils between Homo and Pan dating from 6–7.5 mya.

Rebecca L. Cann is a Professor of Genetics at the University of Hawaii, Manoa. Her interests include human evolutionary genetics and the molecular conservation genetics of endangered species. She is interested in the shared properties of modern endangered species and the early stages of human evolution. These include small population sizes, gender differences in behaviour, infectious disease risks, and geographical isolation.

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